The Present invention generally relates to the processing of crude coal tar to separate and recover valuable chemicals and compositions therefrom. More particularly, the present invention relates to a process for obtaining an increased yield of coal tar pitch from the distillation of a crude coal tar material, and to the pitch product thus obtained.
As general background, the destructive, dry distillation or carbonization of coal yields a liquid condensate, most of which is coal tar. Usually, the coal tar amounts to some 3% of the coal, and is a dark, thick liquid or semi-solid at ambient temperatures. This coal tar includes a mixture of aromatic hydrocarbons such as benzene, toluene, naphthalene, anthracene, xylene and others, phenol bodies such as phenol, cresols, xylenols and others, ammonia, and pyridine and other organic bases, etc.
Upon redistillation of coal tar, several aromatic fractions are obtained usually boiling at temperatures up to about 360.degree. C. These fractions can be further processed by distillation, extraction or other techniques to recover individual or mixed components in a more purified form.
As an example, in an initial phase, ammonia and other gases are separated from crude tar after which the tar is distilled in an operation termed "topping" to separate a first fraction containing certain chemical substituents from higher boiling, more viscous constituents. The distillate from topping commonly termed "chemical oil", has an upper boiling point of about 250.degree. C. and contains tar acids (i.e. the phenolic bodies), naphthalene and tar bases including pyridine and others.
The tar acids usually constitute about 1 to 5% by weight of the crude coal tar, and can be recovered by extraction of the chemical oil with aqueous alkali, e.g. a caustic solution. The aqueous layer is separated from the acid free oil, whereafter the phenols are reconverted to crude form by acidification of the aqueous solution (termed "springing"). The crude phenolics may then be fractionated to obtain phenol, cresols, and higher boiling phenolics such as xylenols.
Further fractions commonly taken in the distillation of coal tar are sometimes called "creosote" fractions or heavy oils, while coal tar pitch is recovered as a residue left behind from such a coal tar distillation.
This residual coal tar Pitch has itself proven to be a highly valuable item of commerce. Coal tar pitch is used for many purposes including, importantly, its use as a binder for carbon and graphite in the formation of carbon bodies and can, for example, be used in the production of anodes, cathodes, electrodes, etc. for use in the metal industry. For example such electrodes are used in electrolytic reduction processes such as aluminium reduction.
Historically, the processing of coal tar is a high volume industry and improvements which provide increased material yields from coal tar distillations while not adversely or unacceptably affecting other aspects of the rectification are highly valued.